E-UTRAN is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. In E-UTRAN mobile networks, the network controls a UE to perform measurement for intra/inter-frequency mobility by using broadcast or dedicated control. For example, in RCC_IDLE state, a UE shall follow the measurement parameters defined for cell reselection specified by the E-UTRAN broadcast. On the other hand, in RCC_CONNECTED state, a UE shall follow the measurement configurations specified by radio resource control (RRC) directed from the E-UTRAN (e.g., as in UTRAN MEASUREMENT_CONTROL).
In single-carrier mobile networks, intra-freq measurement occurs when the current and the target cell operate on the same carrier frequency. FIG. 1A (Prior Art) illustrates an intra-freq measurement scenario. In the example of FIG. 1A, UE11 is served in a current cell on carrier frequency fc1, while the to-be-measured target cell also operates on carrier frequency fc1. In such a scenario, UE11 should be able to carry out such measurements without measurement gaps. This is because the receiver on UE11 is able to measure reference signals on fc1 while simultaneously performing data communication on the same fc1.
On the other hand, inter-freq measurement occurs when the target cell operates on a different carrier frequency as compared to the current cell. Similarly, inter-RAT (Radio Access Technology) measurement occurs when the target cell operates on a different RAT as compared to the current cell. FIG. 1B (Prior Art) illustrates an inter-freq measurement scenario. In the example of FIG. 1B, UE12 is served in a current cell on carrier frequency fc1, while the to-be-measured target cell operates on a different carrier frequency fc2. In such a scenario, UE12 should not be able to carry out measurements without measurement gaps. This is because the receiver on UE12 needs to switch to fc2 to perform measurements and then switch back to fc1 to perform data communication.
In multi-carrier mobile networks with carrier aggregation, it is with high probability that a UE is equipped with multiple receivers. Whether the UE needs measurement gap for inter-frequency or inter-RAT measurement depends on the use of its receivers. FIG. 2A (Prior Art) illustrates a scenario where no measurement gap is needed. In the example of FIG. 2A, receiver A is not configured with any component carrier (CC), while receiver B is used to perform data communication over an active CC on fc2. Because receiver A is idle, it can be tuned to fc3 of the target cell, thus UE21 does not need measurement gap to perform measurements for the target cell. FIG. 2B (Prior Art) illustrates a scenario where measurement gap is needed. In the example of FIG. 2B, UE22 uses receiver A to perform data communication over an active CC on fc1, and uses receiver B to perform data communication over another active CC on fc2. Because both receivers are occupied, UE22 needs measurement gap to perform inter-frequency measurements for the target cell, which operates on a different carrier frequency fc3.
In LTE/LTE-A systems, a UE may report the need for measurement gap (MG) to its serving eNB as part of UE capability. Based on the UE capability report, the eNB configures for MG on per-UE basis. In general, in a first scenario, if at least one receiver is not configured with a component carrier, then the UE may not need MG for inter-freq/inter-RAT measurements. In a second scenario, if all receivers are occupied for data transmission, then the UE may need MG for inter-freq/inter-RAT measurements. Under current MG reporting and configuration, however, the eNB has no information to distinguish between the first and the second scenarios. Besides, with current MG signaling, MG configuration is per-UE basis. Once MG is configured to a UE with multiple receivers, the network does not expect data transmission on all receivers of the UE. In other words, all receivers in the UE should apply the same MG configuration, causing significant UE throughput decrease. On the other hand, if a UE is not configured with MG (but the UE actually needs MG to perform measurements), then the UE should perform inter-freq/inter-RAT measurements only in DRX mode, causing non-up-to-date measurement results and possibly handover delay.
In order to maintain effective and efficient system operation, it is desirable for a UE to accurately report its need of measurement gap to an eNB. It is also desirable for the eNB to properly configure the UE with measurement gap according to its need.